The invention relates to a method for producing heavy-duty components from α+γ TiAl alloys, especially components for aircraft engines or stationary gas turbines.
TiAl-based alloys belong to the group of intermetallic materials, which were developed for uses at temperatures at which super alloys are used. With a density of about 4 g/cc, this new class of alloys offers a considerable potential for weight reduction and, in association therewith, a reduction in stresses of moving components at temperatures up to above 700° C. This weight and stress reduction acts exponentially also on the buckets and blades of gas turbines or, for example, of components of piston engines. The difficulty of processing TiAl alloys by shaping processes is based on the high yield points as well as the low fracture toughness and ductility at low and moderate temperatures. Shaping processes must therefore be carried out at high temperatures in the region of the α+γ or α phase areas under an inert atmosphere.
U.S. Pat. No. 6,110,302 discloses α+γ titanium alloys. Among other things, turbine blades for aircraft engines are dealt with. The use of alloys with about 70% titanium is preferred, the forging temperature being between 815° C. and 885° C. The forging, forming such products as turbine blades, is to have β+α−β regions of different microstructure. Practical investigations have shown that turbine blades, produced according to this method, do not satisfy the requirements in the operating state, especially with regard to the desired fatigue strength.
U.S. Pat. No. 5,593,282 discloses a rotor, which can be used in engines and may be formed, preferably, from a lightweight construction material, in this example from a temperature-resistant ceramic material or, alternatively, from TiAl or NiAl materials.
In the DE-C 43 18 424, a method is described for producing molded objects from alloys based on titanium and aluminum. A cast preform with a lamellar structure with a thickness of up to 1 μm is produced. This is shaped at a temperature ranging from 1050° C. to 1300° C. with a high degree of deformability, so that a dynamic recrystallization with particle sizes up to 5/μm takes place. Subsequently, the preform is cooled and shaped superplastically at temperatures ranging from 900° C. to 1100° C. at rates of 10−4/s to 10−3/s to molded objects having almost the final dimensions. The very fine-grained structure addressed is produced, for example, by the addition of up to 0.3% by weight of silicon. However, this proportion of silicon leads to undesirable side effects, such as an increased porosity and the formation of silicides, as a result of which the mechanical stressability is affected greatly. The fine-grained structure, required for this superplastic shaping is to be brought about by extrusion molding, which does not, however, lead to the finely crystalline, equiaxial structure, which is described elsewhere and required for the superplastic shaping. The extent, to which components, which can be stressed highly mechanically, can actually be produced by this method, is unknown, since this method has not yet gained acceptance in practice.
On the basis of the shaping factors, shown here, the manufacturing methods, addressed in the state of the art and intended, for instance, for TiAl components, do not lead to the technical quality properties required for components, which can be highly stressed dynamically and thermally.